linux-kernel - Re: [PATCH 2/5] KVM: Bail from the dirty ring reset flow if a signal is pending

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Message-ID: <Z4ab_WPpOqhub5uR@google.com>
Date: Tue, 14 Jan 2025 09:16:45 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Peter Xu <peterx@...hat.com>, Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH 2/5] KVM: Bail from the dirty ring reset flow if a signal
 is pending

On Tue, Jan 14, 2025, Yan Zhao wrote:
> On Mon, Jan 13, 2025 at 07:48:46AM -0800, Sean Christopherson wrote:
> > On Mon, Jan 13, 2025, Yan Zhao wrote:
> > > On Fri, Jan 10, 2025 at 05:04:06PM -0800, Sean Christopherson wrote:
> > > > Abort a dirty ring reset if the current task has a pending signal, as the
> > > > hard limit of INT_MAX entries doesn't ensure KVM will respond to a signal
> > > > in a timely fashion.
> > > > 
> > > > Fixes: fb04a1eddb1a ("KVM: X86: Implement ring-based dirty memory tracking")
> > > > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > > > ---
> > > >  virt/kvm/dirty_ring.c | 3 +++
> > > >  1 file changed, 3 insertions(+)
> > > > 
> > > > diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
> > > > index 2faf894dec5a..a81ad17d5eef 100644
> > > > --- a/virt/kvm/dirty_ring.c
> > > > +++ b/virt/kvm/dirty_ring.c
> > > > @@ -117,6 +117,9 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
> > > >  	cur_slot = cur_offset = mask = 0;
> > > >  
> > > >  	while (likely((*nr_entries_reset) < INT_MAX)) {
> > > > +		if (signal_pending(current))
> > > > +			return -EINTR;
> > > Will it break the userspace when a signal is pending? e.g. QEMU might report an
> > > error like
> > > "kvm_dirty_ring_reap_locked: Assertion `ret == total' failed".
> > 
> > Ugh.  In theory, yes.  In practice, I hope not?  If it's a potential problem for
> > QEMU, the only idea have is to only react to fatal signals by default, and then
> > let userspace opt-in to reacting to non-fatal signals.
> So, what about just fatal_signal_pending() as in other ioctls in kernel?

Ya, though I would leave the decision up to Peter or Paolo (or someone else that
knows what QEMU wants/prefers/tolerates).

> if (fatal_signal_pending(current))
> 	return -EINTR;